Owing to their exceptional properties, notably their advantageous weight-to-resistance ratio, composite materials find numerous applications in the field of cycle construction, in particular cycle frames, and allow a substantial weight reduction, so that aerodynamical shapes for the individual bars or tubes of the frame may be achieved.
The front fork is however hardly suitable to the use of these materials. Indeed, conventional forks made of metal or light metal alloys terminate at their upper end by a cylindrical fork head which passes through a steering tube provided in the frame and is supported for rotation by means of two ball bearings at the two ends of the steering tube. The handlebar is fixed at the top of this fork head by means of clamping members. Since the rotation axis of the wheel must be located forwardly of the steering axis for stability reasons, the arms of the fork have a curved shape with a forwardly oriented concavity.
Since this structure has been confirmed for a long time because of its simplicity and its efficiency, it has simply been retained during the first attempts to realize the fork with composite materials. These attempts have however led to relatively bad results. In fact, considerable flexion torques are exerted on the cylindrical head at the top of the fork, most particularly at the height of the lower bearing of the associated steering tube or head. When dealing with a fork head made of metal, it is easy to select the metal and the wall thickness, in order to take up these flexion torques without deformation. On the contrary, when dealing with composite materials, it is extremely difficult to achieve a sufficient moment of inertia without detrimentally increasing the outer diameter of the fork head.
In order to remedy some of these drawbacks, U.S. Pat No. 4,008,903 discloses a front fork made of molded plastic material comprising two fork arms which unite to form an upper fork head, the latter comprising two rearwardly directed projections, an upper projection and a lower projection. These projections both carry pivot portions which cooperate with complementary pivot portions of the steering head of the frame which define a steering axis. However, this document does not teach how the rigidity of the fork head is ensured. Further, the outline of this fork head has numerous reliefs which are prejudicial to the aerodynamism of the cycle.
It is finally noted that in the front fork shown in U.S. Pat. No. 4,008,903, the fork arms are curved forwardly such that the axis of a wheel attached at their ends is forwardly off-set with respect to the steering axis, by the distance necessary for ensuring the stability of the cycle in a known manner.
While it is easy to make the curved fork arms of metal, the manufacture of such curved fork arms is significantly more difficult when using composite materials as the orientation of the reinforcement fibres must be very precise in these materials, which however cannot be guaranteed in all circumstances because of the lack of precision of the manual insertion. For economical reasons, it is almost impossible to carry out an automated insertion, and for these reasons, it is thus usual to continue to make these forks at least partly of metal.
Besides, it is desirable to mount the handlebar with possibility of adjustment both upwardly/downwardly as well as in the forward/rearward direction in order to be able to take into account the body dimensions of the cyclist as well as the particular conditions of a circuit, notably during a cycle race. To this end, it is known to provide devices with two telescopic arrangements, a vertical one and a horizontal one, which can be easily realized with metal but not with composite materials.
In certain cases, in particular for cycle races, the handlebar is lowered down to below the height of the upper bearing of the steering tube, in fact by means of bent pieces which are attached at the top of the fork head and extend downwardly. These devices are complex and necessitate numerous preparation steps and the exchange of components before each corresponding adjustment or adjustment modification.